Changing tire sizes (newbie question)

Vehicle is a '76 Porsche 914, and all four tires are completely dry rotted. I need to get this roadworthy for a cross country trip coming up real soon so I'm looking to replace the tires. But the catch is I can't afford $150+ per tire right now so looking to do this as cheaply as possible.

How much wiggle room do I have for cross section and aspect ratio?

Existing tires:
Front: P205/60 R15
Back: P225/50 R15

Those 225/50/15's are quite a bit more than I'd like to spend. Is there any reason I can't just put 205/50's on there? Or even 205/60 if I measured the vertical clearance in the rear first?

Or maybe I could go even more narrow, if there's no technical reason they _need_ to be that wide for grannie-type driving? Just not understanding what I can and cannot do here in terms of sizes. How narrow can I go? And again I'm looking for super-cheap tires here, just to get the thing able to make a 2000 mile round trip at legal speeds.

You can get into a book of argument about original design and handling. That is probably true to a fine tuned point. In your case, unless you're building it up for a show car or a collectors, I would put a full set of all season 205/60 (or 65 s if you have the clearances).

As a rule the lower sidewalls have less lateral movement = better high speed handling. Mixing tire sizes front to rear can be argued to cause a difference in handling as sidewall flex varies with tire sizes = one end moving more than the other. If your Porsche came out with the same tire all the way around, I would go that route with some $65.00 - 40,000 mile tires. Keep in mind you may have a speedo reading change with the tire differences.

Also, lower profile tire will hydroplane more than a higher one, will ride stiffer, and wheels more subject to damage by hitting pothole edges. Might try tirerack.com for further info and pricing. They sent my tires next day (cheaper than anywhere local even after including installation costs) which surprised the dickens out of me.
B

Ok, so skinnier tires have a few drawbacks. My car-knowitall friend said I "may" be able to put 165/175's on it, which was kind of the reason for my post. I don't know how to tell if the rims are compatible with something that skinny. But heck saving just $20 a tire isn't worth it I guess.

marbobj-

Definitely not for competition or anything like that. Glad to know that I don't _need_ to go with the wider tires on the back. I do have one question about your suggestion though:

The back currently has 50's on it, but you recommended 60's (or even 65) all around. But then you also said that the lower sidewalls (50?) have better handling performance. Was this just a cost-saving suggestion for my benefit?

Other thing though, I haven't yet measured the clearance back there. Worse yet, the car will be _heavily_ loaded aft for this trip, and after all these years I'm sure the springs are soft. How do I know I'll be 100% safe with taller tires back there?

I suppose I could just throw a couple hundred pounds back there before I measure. Yeah that's just what I'll do.

And yep, gotcha on the speedo reading. That I can calculate. But hehe it doesn't matter because the speedo doesn't even work. LOL. I always take a GPS along for that purpose because none of the speedometers are correct on any of my Porsches.

As for the condition, well it's in reasonable shape visually. It's been sitting in the garage for 7 years after I lost my license. Plus, well, shame for me to say it but I just don't love driving it like I do my 944. The things of course shift like a VW, and you really have to actually think about things rather than just being ONE with the car. One of the flares is cracked but that's about it.

Mechanically I don't trust it yet. That's why I need tires pronto, so I can get it on the road and see what's up with it.

Yeah, Core that's just from the money end of things. The 60 and 65 s are usually a little cheaper. Probably 'cause they're used more in the 15 inch rims. The 165 and 175 s - I'm pretty sure you'll be looking at a rim change. That's assuming the rims on it now are matched to the size tires that are on it.

you need to be concerned with weight capacity of the tires. a 165/50 will carry much less weight than a 205/50. installing a too small of tire can become a safety hazard if the weight capacity is not great enough.

you also need to be concerned with the rim width. too wide of a rim on a skinny tire can cause a lot of problems (as in crash).